The present invention relates to a permanent magnet-embedded rotor-type motor.
In recent years, various motors have come to be developed, but the application of permanent magnet motors has been carried out extensively because of the high energy density and efficiency. Further, the development of embedded magnet-type motors, in which permanent magnets arranged in a rotor are embedded in a core, is underway. The embedded magnet-type motor realizes a high torque since permanent magnets are effectively arranged in the core to provide saliency in a magnetic circuit in the rotor, but it has been thought that the leakage of the magnetic flux to outer peripheral rib portions is unavoidable for the reason of mechanical strength. The outer peripheral ribs must be made thin to reduce the leakage of the magnetic flux. On the other hand, however, to effect high-speed rotation, it is necessary to secure mechanical strength by making the outer peripheral ribs thick. The above-described conflicting characteristics have been detrimental to the realization of higher-speed, higher-torque motors.
A related-art open trapezoidal shaped rotor is shown in FIG. 8. Reference numeral 1 denotes a stator core formed in a hollow cylindrical shape, and slots 2 are punched on its inner side in a number determined by such as the number of poles and the number of phases, coils 3 being wound between the slots 2. Numeral 4 denotes a cylindrical rotor core disposed on the inner side of the aforementioned stator core 1, and open trapezoidal shaped magnet insertion holes 5 which extend through in the axial direction are formed in it.
Reference numeral 6 denotes a magnetic-flux holding portion which is located in an outer peripheral portion of the rotor core 4 and is sandwiched by inner edges of the open trapezoidal shaped magnet insertion hole 5. Numeral 7 denotes a yoke portion surrounded by outer edges of adjacent ones of the open trapezoidal shaped magnet insertion holes 5. Numeral 8 denotes a connecting portion for connecting the magnetic-flux holding portion 6 and the yoke portion 7 and disposed on the outer side of the magnet insertion hole 5 adjacent thereto. The connecting portion 8 has a width necessary for securing the strength.
Reference numeral 9 denotes a permanent magnet inserted in the magnet insertion hole 5. The permanent magnets 9 are embedded in the open U-shape which is symmetrical with the magnet insertion hole 5 and divided into three parts, and form one magnetic pole. Further, polarities of the permanent magnets 9 of adjacent poles a rearranged to be of opposite polarities. Reference numeral 10 denotes a rotor-shaft inserting hole.
The related-art rotor of the open trapezoidal type is constructed as described above, and assuming that the coils 3 are three-phase windings, if a three-phase alternating current is allowed to flow across the coils 3, the rotor core 4 rotates as there occur the torque occurring due to the current flowing across the coils 3 and the magnetic flux of the permanent magnets 9, i.e., the permanent magnet field torque, as well as the torque in which the magnetic flux occurring due to the current flowing across the coils 3 tends to move to a magnetically stable position, i.e.; the reluctance torque. Namely, the construction is such that the magnetic flux due to the permanent magnets 9 is related to the torque generated by the motor.
With the open trapezoidal-type rotor of such a related-art permanent magnet motor, since the divided permanent magnets 9 are embedded, the centrifugal force acting in the permanent magnets and the centrifugal force acting in the magnetic-flux holding portions 6 surrounded by the magnets of the open trapezoidal type are concentrated in the connecting portions 8 of the rotor core. For this reason, the radial width of each connecting portion 8 must be relatively large in order to maintain the strength.
Accordingly, since the radial width of each connecting portion is relatively large, the leakage flux occurring in the connecting portions 8 increases, so that the magnetic flux which is interlinked with the coils decreases. Hence, there occurs the problem that the energy density of the motor declines.
For this reason, if the magnets at both outer peripheral portions of the open trapezoidal portion are made long by sacrificing the strength so as to increase the magnetic flux in the open trapezoidal portion, the number of revolution can be set to only 6000 rpm or thereabouts so as to maintain the centrifugal force within a range in which the radial portions of the connecting portions 8 are not broken. Hence, it has been impossible to realize an embedded magnet-type motor capable of high-speed rotation and high-torque at same time.